1. Field of the Invention
The present invention relates to mechanisms for extracting water from a web of material, and more particularly from a fibrous web being processed into a paper product on a papermaking machine. Specifically, the present invention is an impermeable belt designed for use in conjunction with an extended nip press on a papermaking machine.
2. Description of the Prior Art
During the papermaking process, a fibrous web is formed on a forming wire by depositing a fibrous slurry thereon. A large amount of water is drained from the slurry during this process, after which the newly formed web proceeds to a press section. The press section includes a series of press nips, in which the fibrous web is subjected to compressive forces designed to remove water therefrom. The web finally proceeds to a drying section which includes heated dryer drums around which the web is directed. The heated dryer drums reduce the water content of the web to a desirable level through evaporation.
Rising energy costs have made it increasingly desirable to remove as much water as possible from the web prior to its entering the dryer section. The dryer drums are often heated from within by steam and related costs can be substantial, especially when a large amount of water needs to be removed from the web.
Traditionally, press sections have included a series of nips formed by pairs of adjacent cylindrical press rollers. Recently, the use of extended press nips has been found to be advantageous over the use of nips formed by pairs of adjacent rollers. The longer the web can be subjected to pressure in the nip, the more water can be removed there, and, consequently, the less will remain to be removed through evaporation in the dryer section.
The present invention relates to extended nip presses of the shoe type. In this variety of extended nip press, the nip is formed between a cylindrical press roller and an arcuate pressure shoe. The latter has a cylindrically concave surface having a radius of curvature close to that of the cylindrical press roller. When roller and shoe are bought into close physical proximity, a nip is formed which can be five to ten times longer in the machine direction than one formed between two press rollers. This increases the so-called dwell time of the fibrous web in the extended nip while maintaining the same level of pressure per square inch pressing force used in a two-roller press. The result of this new extended nip technology has been a dramatic increase in dewatering of the fibrous web in the extended nip when compared to conventional nips on paper machines.
An extended nip press of the shoe type requires a special belt, such as that shown in Canadian Patent No. 1,188,556. This belt is designed to protect the press fabric supporting, carrying, and dewatering the fibrous web from the accelerated wear that would result from direct, sliding contact over the stationary pressure shoe. Such a belt must be made with a smooth impervious surface that rides, or slides, over the stationary shoe on a lubricating film of oil. The belt moves through the nip at roughly the same speed as the press fabric thereby subjecting the press fabric to minimal amounts of rubbing against stationary components.
The normal configuration for an extended nip press involves the use of two press fabrics. The components in the nip can be listed in the following sequence: a grooved cylindrical press roller, a first press fabric, the fibrous web or nascent paper sheet, a second press fabric, the special belt, the lubricating film, and the arcuate pressure shoe.
All of the extended nip presses currently in commercial operation are of the above configuration. However, this so-called double-felted extended nip press is limited to certain types of paper grades. To expand the application of the extended nip press to finer paper grades and newsprint, it will be necessary to develop a single-felted extended nip press.
In a single-felted extended nip press, the components in the nip would be listed in the following sequence: a smooth cylindrical press roller, the fibrous web or nascent paper sheet, the press fabric, the special belt, the lubricating film, and the arcuate pressure shoe.
In the double-felted extended nip press, the water pressed from the fibrous web can pass into either of the two press fabrics, as well as into the grooves provided in the cylindrical press roller. In the single-felted extended nip press, two of these sinks for water storage have been eliminated. There is only one felt and, because the fibrous web contacts the cylindrical press roller directly, this roller must have a smooth surface. Clearly, this loss of temporary water storage space must be replaced if a single-felted extended nip press is ever to achieve widespread use. It is absolutely mandatory that voids be provided in the nip of the press, so that water pressed from the paper sheet in a single-felted extended nip press will have a place to be stored.
Two approaches have been taken to achieve a satisfactory solution of this problem. Canadian Patent No. 1,190,779 shows an extended nip press belt having voids on the felt side. These voids arise from the weave pattern of the base fabric and the absence of impregnation on the felt side of the belt, and provide a place into which liquid can be transferred during passage of the fibrous web, press fabric, and special belt through the nip.
The other approach is represented by U.S. Pat. No. 4,559,258 and 4,643,916. These show an extended nip press belt having grooves on the felt side to store water pressed from the fibrous web.
While these two approaches were directed primarily toward achieving a solution to the water-removal problem in a single-felted extended nip press, it has been found that use of either belt in a double-felted extended nip press leads to enhanced dewatering.
In practice, however, it has also been found that satisfactory grooved ENP belts cannot be produced for commercial use. All attempts to make this structure to date have resulted in a product which fails due to delamination of grooves, and/or collapse of the grooves. The present invention is a special belt for an extended nip press which would not be susceptible to delamination.